Having a system for moving a shuttle element on a surface accurately, with high speed and in a selected orthogonal plane is highly desirable.
This invention relates to linear motors for moving a shuttle element over a plane in an orthogonal manner. In particular, the invention relates to AC linear induction motors for moving the element on the plane. The invention is also concerned with controlling the movement of single axis linear induction motor on a planar surface.
Different systems have been developed for moving a shuttle element over a plane in an orthogonal pattern. One of these systems is that disclosed in U.S. Pat. No. 3,376,578 (Sawyer). This system operates with four motors incorporated in a shuttle element moving over a magnetic platen. These motors are variable reluctance motors or stepping motors which are open loop. There is a limit on the amount of force which can be generated in such motors and therefore a limitation on the amount of movement that can be obtained from such motors. There is also no feedback applied to such motors. As a result, the open loop system is one which depends on the accuracy of the platen machining. There are therefore substantial limitations in providing an effective motion control.
An alternative way of moving a shuttle element over a surface is that described in U.S. Pat. No. 4,654,571 (Hinds). In this system, two DC motors are used to control the movement of the shuttle over a platen. There is a closed loop system in that the platen is formed in a checkerboard pattern of oppositely magnetized permanent magnetic elements with which the motors and the platen can interact. A laser is used at the perimeter of the board for interacting with the platen so as to provide the feedback closed loop to control movement of the motor over the platen. This is a relatively complex system.
It is also disclosed in U.S. Pat. Nos. 5,648,690 and 5,763,966, to have a system of two separate DC motors arranged in orthogonal relationship with each other to move a separate shuttle over a platen. Air bearings are used between the shuttle and the DC motors so that the shuttle is substantially independent of the motors. The contents of that application by the present applicant are incorporated by reference herein.
It is known to have vector control of induction motors rotatable about an axis. This has been described in "AC Motors for Servo Positioning" by John Mazurkiewicz et al. in Motion Control, November/December 1991, pages 36-39; "Principles of Vector Control, Parts I and II" by D. Ohm in PCIM, August 1990, pages 41-43 and PCIM; pages 32-36 and "Understanding AC Vector Drives" by Webb et al. in Power Transmission Design, February 1993, pages 17-19.
An AC induction motor, as opposed to the typical DC servo motor, has a non-linear speed vs. torque curve. Also, there are three variables impacting this torque curve compared to essentially one variable in the DC motor case. The AC motor provides a fairly flat speed vs. torque curve, up to the point where the "slip" of the motor begins to exceed a certain percent of the rotating frequency of the motor. Then the speed falls off rapidly, actually resulting in a function that contains two values of speed over a certain torque range (and increasing slip) as the speed finally falls to zero. This condition results in poor torque behavior at zero motor speed. This makes the motor unusable in a positioning system, a classical servo application.
On the other hand, the AC induction motor has many aspects to recommend it for system use. It is usually less expensive, and more reliable, than an equivalent power DC motor because of its less complex construction. Furthermore, as motor size increases into the 1 horsepower and up region, these advantages increase to the point where a DC motor is not competitive economically. So it has been increasingly desirable to find the means to control the AC motor in a manner to overcome its poor static and nonlinear dynamic behavior.
Over time, the equations that govern the AC motor behavior have become well-known. A problem has been the prohibitive cost to implement a control that could execute in real time. With the improvement in feedback transducers and computing power at an attractive price, it has become feasible to provide a level of control that effectively forces the AC motor to behave as though it had the same characteristics as the DC motor.
A known control method for rotational motors is known as vector control. This method employs internal vector control algorithms to control the torque more effectively. This is accomplished by controlling the air gap flux, rotor slip, and total stator current. In order to control these variables, the following motor parameters need to be measured: armature and field current, rotor speed, and temperature. These measured parameters are operated on by a fast processor in order to calculate the voltage to be applied to the armature and field. The processor may be required to perform many thousands of iterations per second in order to control the motor behavior in real time.
In operation, the field current is set by a flux command. The armature is set by the outer loop feedback. This measures the difference between the reference speed and the actual speed to produce the required torque. The stator current is determined by calculating the vector sum of the field and armature current. By modulating the drive voltage applied to the motor, the vector relationship between the rotor speed and slip can be controlled. When this relationship is fixed over the motor speed range down to the stall speed, the resulting behavior of the AC motor vs. voltage command is the same as that of a DC motor.
There are other equations that can be implemented to obtain the net result described above. There are tradeoffs in what is measured and the degree of control achieved. Some of these methods require more precise measurement of certain motor parameters and the precision of control can vary according to the equation being implemented. This is true for linear, as well as rotary, AC induction motors. From a modeling consideration, there is little difference, mostly second-order effects due to the mechanical differences in construction.
There is a need to provide a system for moving an element in the form of a shuttle over a surface in an orthogonal manner using AC induction motors in a manner which minimizes the disadvantages of known systems and uses the advantages of AC induction motors. There is also a need for accurate control of induction motors moving along a single axis on a planar surface.